While contemplating "magic is powerful", I was trying to figure out just how many spells can break the game. I decided to go through the Spell Compendium until I found an overpowered spell. Sure enough, the second spell printed in that book can be exploited. The spell Absorption (SpC) can easily be used to give you infinite spells!
Not running out of spells is of course very powerful. You can nova at will, face many encounters per day without private demiplane cheese, and so on. For example, my character wants to build a castle as a base of operations, e.g. I want
this 10 floor castle . However I don't want to spend the large fees in the Stronghold Builder's Guide. Instead I want to spam Wall of Stone and construct most of my castle for free with magic. Unfortunately, even at caster level 20, it will take about 20000 castings of Wall of Stone to create the volume of stone needed. Spending all level 5+ slots on casting Wall of Stone every day will take about two years, where I want to create my castle in just a day or so.
Absorption is a spell that absorbs spells or spell-like abilities targeted at you, giving you a separate set of what amounts to psionic power points to use to cast your own spells. Each spell absorbed gives you one point per spell level, and spells can be cast using one point per level. For example, if you absorb a Disintegrate (level 6), you can cast Fireball (level 3) twice for free.
One casting of Absorption will absorb 6+1d4 levels of spells. What you want to do is Maximize it when cast, which will absorb 10 spell levels. Once 10 levels are absorbed and you have 10 points, use 9 of them to cast Absorption again and continually repeat the process. Each time through gives you another free point which can be used in casting other spells for free. For extra speed, apply additional metamagic if possible such as Empower, Twin, and/or Repeat spell. With psionics it's easy to see infinite power point loops, however techniques for infinite arcane spells don't seem to be as well known.
Absorption is a 9th level spell, and Maximize is +3 level, so casting it Maximized many times in a row requires certain class features. For example, take the feat Arcane Thesis (Absorption), and two +0 slot metamagic feats (such as Invisible spell and Sanctum spell), to cast it Maximized in a 9th level slot. Alternatively you can take one level in the Spelldancer PrC (which annoyingly has a four feat entry tax) along with a means to be immune to exhaustion and ability damage.
To absorb spells, you need to have a class feature, item, or ally that can spam a spell or spell-like ability on you, to charge your spell points. For example with the Half Fey template you can Charm Person yourself 10 times in a row, or with a Ring of Telekinesis you can do violent thrust on yourself twice. My favorite way is to cast Call Faithful Servants (BoED) to bring in Musteval Guardinals that stay with you for a year. Once you have 10 of them, have them all ready actions to cast their at will Magic Missile on you the moment you cast Absorption, so you get your spell points with no delay. Repeating the process gives you one spell point per round, or a free 9th level spell every 54 seconds.
The Red Wizard's Handbook has the good idea of putting Absorption inside a casting of Energy Transformation Field (SpC). That has the downside of costing 5000 gold and 250 XP, however once set up you effectively get Absorption cast (and hence another 9 spell points) for free. If Absorption is cast for free, you also don't need to Maximize it. However the Maximized Absorption route has the benefit of being completely portable with no gold/XP costs.